The invention relates generally to optical switching elements and more particularly to a system and method for manipulating optical signals by operations within an optical switch.
Optical signals can be selectively rerouted by optical switches used in both telecommunication systems and data communication systems. In circuit switching, any one fiber in an array of parallel input optical fibers may be connected to any one fiber in an array of output optical fibers using a matrix of optical switches. As a result, an incoming data packet from a particular input fiber can be directed to a selected output fiber, based upon the destination of the packet.
U.S. Pat. No. 4,988,157 to Jackel et al. describes a bi-stable optical switching arrangement utilizing electrochemically generated bubbles. Parallel input waveguides and parallel output waveguides are formed on a substrate at perpendicular angles so as to intersect. A 45 degree slot is formed across each intersection. The slots are selectively filled with a fluid, such as water or a refractive index-matching fluid. Electrodes are positioned adjacent to the slots and are selectively activated to electrolytically convert the fluid to gaseous bubbles. The electrolytic formation of the bubbles destroys any index-matching properties across the slots and causes light to be reflected at the slot sidewall. Thus, an electrolytically formed bubble within a particular slot results in the reflection of optical signals at the slot, rather than the propagation across the slot. The presence of either a catalyst, an electrical pulse of opposite polarity, or an electrical pulse of significant size and of the same polarity will collapse the bubble, thereby returning the switch to a transmissive state.
Although the approach taken by Jackel et al. is simple and potentially inexpensive in large quantities, and achieves a number of advantages over prior approaches, further improvements may be realized. Where water is used as the fluid, electrolysis generates H2 and O2 bubbles in order to create a reflecting state, but the water provides a poor index match to the waveguides when the switch is returned to the transmissive state. Consequently, crosstalk is high if water is used. Another concern is that the bubble-creation process and the bubble-removal process may be too slow to meet the desired transition time for telecommunication switching.
U.S. Pat. No. 5,699,462 to Fouquet et al. provides a more promising approach. The switching arrangement of Fouquet et al. utilizes electrically driven heaters as a means for controlling the direction of the optical signals transversing the switch. Intersecting input waveguides and output waveguides are formed on a silicon substrate. In the transmissive state, an index-matching fluid fills the intersection, enabling light to continue in the input waveguide direction. To initiate switching from the transmissive to the reflective state, a heater is selectively energized to form a bubble within the intersection between the input and output waveguides. The formation of a bubble destroys any index-matching properties across the intersection, resulting in the reflection of optical signals away from the input waveguide direction. A concern with the heater approach is that there is a loss of heat to the surrounding silicon substrate, which necessarily increases the power requirement to create and hold the bubble in place. Another concern is that there may be significant xe2x80x9cthermal crosstalkxe2x80x9d among switches on a single silicon substrate as the temperature in the surrounding silicon substrate is elevated by heating at one of the switches.
What is needed is a switching element and an arrangement which enable reliable and repeated transitions between transmissive and reflective states, thereby controlling optical communication between optical signal lines.
The invention is a system and method for controlling optical signals in an optical switch using a piezoelectric actuator. The optical switch comprises: (1) a first input waveguide and a first output waveguide with ends at an intersecting gap, (2) a trench having walls for accommodating a fluid, wherein the trench includes the gap, and (3) the piezoelectric actuator positioned along a wall of the trench. In response to an input voltage, a movable membrane of the actuator in the preferred embodiment is configured to switch between an outward position and an inward position in relation to the interior of the trench. In other embodiments, the movable membrane is configured to switch among an outward position, an inward position and a relaxed position in relation to the interior of the trench. The membrane is positioned relative to the gap such that the displacement of the membrane to one of the selected positions results in the manipulation of the fluid disposed within the gap. In one position, the gap is filled with an index-matching liquid that causes light from the first input waveguide to transmit through the gap and into the first output waveguide. In another position, the gap is filled with a gaseous bubble that creates a refractive index mismatch that causes light from the first input waveguide to be diverted at the gap. Preferably, the diversion is in a direction of a second output waveguide. In another embodiment, there are two piezoelectric actuators for manipulating the fluid disposed within the gap. The two actuators are positioned relative to the gap to devise a xe2x80x9cpush-pullxe2x80x9d configuration.
The movable membrane may include a stress-biased lead zirconia titanate (PZT) material for converting electrical fields into mechanical displacements. Utilizing a PZT material for deflection is preferred due to its high range of mechanical deflection per unit of electrical energy input. Other materials with similar piezoelectric properties may also be used.
The piezoelectric actuator includes a first electrode and a second electrode coupled to a voltage source. Each of the electrodes is in contact with the membrane and with the voltage source. In one embodiment, the electrical connection to the first electrode is provided on the side of the membrane opposite to the electrical connection to the second electrode. In another embodiment, the two electrical connections are provided on a same side of the membrane.
In the preferred embodiment, the movable membrane is displaced from a relaxed position to a first position that is either the inward position or the outward position in response to an input voltage. In the inward position, the membrane is in a convex orientation in relation to the wall of the trench, such that the index-matching liquid disposed within the trench is displaced in a direction away from the membrane. Alternatively, in the outward position, the membrane is in a concave orientation in relation to the trench wall such that the index-matching liquid disposed within the trench is enabled to flow in a direction toward a cavity created by the orientation. An input voltage of opposite polarity displaces the membrane to a second position that is opposite from the first position. Maintaining the membrane in one of the two positions requires a continuous voltage input, but the level of input may be reduced. While not critical, an input voltage may displace the membrane from the first or second position to the relaxed position. In other embodiments, the movable membrane is displaced from a first position that is a convex or a concave orientation to a second respective position that is more convex or concave.
At least one gaseous bubble is provided within the liquid-filled trench. An acceptable means for forming the bubble includes degassing a dissolved inert gas contained within the index-matching liquid. Other bubble forming techniques may also be used. Utilizing an inert gas, or a mixture of inert gases, is preferred due to its non-reactive properties with the index-matching liquid. Acceptable inert gases include nitrogen, xenon, krypton, argon, neon, helium, carbon dioxide, sulfur hexafluoride, and the like.
In one embodiment in which there is one movable membrane, a first bubble and a second bubble are provided within the liquid-filled trench. In the convex orientation, the first bubble that is near the membrane is displaced into the intersecting gap to create a refractive index mismatch, resulting in the diversion of the optical signals from the direction of the first input waveguide, so that the signals do not reach the first output waveguide. Preferably, the diversion is in the direction of the second output waveguide. Moreover, since the volume within the trench is reduced by an amount of displacement by the membrane, the first and second bubbles are compressed.
Switching the membrane to the concave orientation allows an expansion of the volume within the trench by an amount of displacement by the membrane, resulting in pressure relief to the compressed bubbles. The compressed second bubble that is farthest away from the membrane functions as a xe2x80x9cmechanical springxe2x80x9d to xe2x80x9cpushxe2x80x9d the index-matching liquid immediately adjacent to the second bubble in a direction toward the membrane, resulting in the displacements of the liquid into the gap and the first bubble away from the gap to allow the propagation of the optical signals from the first input waveguide to the first output waveguide. The exact locations of the bubbles and the index-matching fluid when the membrane is in a selected orientation is not meant to be limiting. That is, the bubbles and the liquid can be in different locations when the membrane is in the same orientation without diverging from the scope of the invention.
In another embodiment in which there are two movable membranes on opposite sides of the intersecting gap, one bubble is provided within the liquid-filled trench. The two membranes are configured to be in opposing orientations such that the bubble is displaced away from the gap when the first membrane is in a first orientation (e.g., concave orientation) and the second membrane is in a second orientation (e.g., convex orientation). Likewise, the bubble is displaced into the gap when the first membrane is in the second orientation (e.g., convex orientation) and the second membrane is in the first orientation (e.g., concave orientation).
While the invention is described as utilizing a membrane for manipulating the index-matching liquid within the gap, other types of actuators, such as a piston, can be used without diverging from the scope of the invention. Moreover, the trench may include a geometric constriction for holding the bubble at a fixed location within the trench once the bubble is displaced into that location by the movement of the membrane. In this arrangement, the level of the input voltage to maintain the bubble at a particular location within the trench may be lessened. Alternatively, the trench is tapered at one end, such that the cross-sectional area at one end is smaller than the cross-sectional area at the other end.
One of the advantages of utilizing piezoelectric actuator(s) in an optical switch is that the power requirements for switching between different orientations and for holding the membrane in place are kept low. Another advantage is that very little heat is dissipated to the surrounding structure, due to the low power consumption by the actuator(s). Consequently, thermal crosstalk among switches is reduced. Other embodiments include different advantages in addition to those described above.